Preparation of photosensitive crystals



Jan. 13, 1959 v QwElNRl-:ICH 2,868,736

PREPARATIQN oF PHoToYsENsITIvE CRYSTALS Fild oct. l18. 1955 AT TO R N EY2,868,736 Patented Jan. 13,v 1959 2,s6s,1s6 PREPARATION oF PHoTosENsmvECRYSTALS Otto Weinreich, East Orange, N. J., assignor to Tung-SolElectric Inc., a corporation of Delaware Application October 18, 1955,Serial No. 541,147

Claims. c (Cl. 252-501) The present invention relates to photosensitivecompounds of thetype that increase in electrical conductivity whensubjected to light rays and comprises a novel method for preparingphotosensitive compounds of cadmium or the like that is adapted forpractice on a commercial scale and results in a product of substantiallyuniform electrical characteristics and of relatively highphotosensitivity.

The process' is applicable to starting compounds which have little or noinitial photosensitivity. Examples of suitable starting materials aresuliides, selenides and tellurides of cadmium, indium, thallium, tin,lead, antimony and bismuth and combinations thereof, such as sullidesand tellurides of silver and thallium combined with silver, bismuth, tinor thallium. It is not necessary that the starting material bechemically pure. In fact superior results have been obtained when thestarting material contains traces of impurities such as are to beexpected in commercial products. The invention is directed primarily tothe process whereby photosensitivity is enhanced in materials of theabove type and not to any new chemical compound.

For convenience in the following description of the process of theinvention, cadmium selenide is specified as the starting material. Itwill `be understood that the process is equally applicable to othermaterials such as above indicated.

In general the process of the invention comprises vaporization of thestarting material under vacuum and condensation upon a metal wall inwhich, or on the surface of which, is the activator for enhancement ofthe photosensitivity of the deposited crystalline material. Theactivator, preferably oxygen, may be adsorbed by the metal wall or itmay comprise part of a coating on the wall. For example, the metal maycontain occluded oxygen, or the metal may be oxidized or may be coatedwith an oxide of another material. vBy control of the amount of oxygenor other activator combined with the metal wall upon which the materialis to be deposited and of the temperature difference between the walland that applied for vaporization of the starting material, the quantityof activating material for combination with the compound may becontrolled to yield optimum photosensitivity.

The activated crystalline material deposited upon the metal wall isreadily removable by mechanical means and is suitable for use inmanufacture of photoelectric cells.

For a better understanding of the invention reference may be had to theaccompanying drawing of which the single gure is a diagrammaticalrepresentation of apparatus suitable for use in practicing the newprocess.

The starting material 2, for example, commercial cadmium selenidecontaining traces of impurities, is placed in a quartz container 4having a loose fitting lid 6 of tantalum, kmolybdenum or other metal ofhigh melting point containing or carrying an activator for the materialto be deposited thereon. A lid of oxidized molybdenum has been foundparticularly satisfactory in the process. The container 4 with itscontents andthe lid 6 is placed 2 within a platinum cylinder 8 about thelower end of which is a coil 10 to which radio frequency current isdelivered through leads 12. A cooling coil 14' is positioned about thelid 6 and temperature-responsive means as, for example, a thermocoupleindicated diagrammatically at 16 is provided for control of thetemperature of the lid. The

entire asesmbly of quartz container 4, platinum cylinder 8, highfrequency heating coil10 and cooling coil 14 is enclosed within achamber 18 adapted to be evacuated as by a connection, diagrammaticallyindicated at 20, to a vacuum system. The chamber 18 is maintained at apressure of about l06 mm. and the temperature of the starting materialraised by radio frequency induction heating to about 900 C. As the coilsurrounds the lower portion only of the platinum cylinder and as the lid6 is maintained at a lower temperature by means of the cooling coil 14there will be a temperature differential maintained between the startingmaterial in theV bottom of the yquartz container and the metal-lid 6.Heating is maintained for about thirty minutes after which time theinside surface of the lid 6 will be found covered with a coating ofcrystals which sticks to the metal lid and may, after cooling, beremoved mechanically. With a quartz cylinder of about 1/2" diameter anda height of 1" one run will form suicient deposit from which up to ftysmall photosensitive cells can be made.

The metal lid 6, instead of being oxidized, may be coated with a thinlayer of barium peroxide or with a thin layer containing an activatorother than oxygen.

The following gives an indication of the results obtaina'ble by practiceof the invention and of the importance of the condition of the metalplate upon which the crystals are deposited. Crystals of commercialgrade cadmium selenide were deposited on a molybdenum plate or lid thathad ben hydrogen fired to remove all occluded oxygen and to clean thesurface and also on a plate of oxidized molybdenum. The deposits on theplates were then removed and the increase in current with equalillumination and equal voltage applied across equally spaced silverpaste electrodes on the crystals from each deposit was measured. Thedark currents of both deposits were of the same order of magnitude butthe currents of illuminated crystals removed from the oxidizedmolybdenum plate were over one hundred times as large as the currents ofilluminated crystals removed from the cleaned plate. When commercialcadmium selenide was deposited on a molybdenum plate coated with bariumperoxide, photocurrents over one thousand times greater than those ofthe deposit on cleaned molybdenum were obtained. The dark currents ofthe deposit from the barium peroxide coated molybdenum were howeversubstantially larger than the dark currents of the deposits on eitherthe oxidized or cleaned molybdenum plates.v

Photosensitivity of crystals of cadmium selenide deposited on oxidizedmolybdenum with 22.5 volts across an electrode spacing of about l mm.and illumination of 270 microwatts per square centimeter in the regionof maximum spectral response (}\=.750;i) averaged over amperes per wattfor four samples.

The above described results obtained when commercial cadmium selenidewas deposited under vacuum on oxide coated molybdenum plates are typicalof those obtainable by the process of the invention irrespective of theparticular starting material and of the particular metal and activatoremployed. Suitable starting materials have heretofore been indicated.Metals suitable for use as the plate upon which the vaporized startingmaterial deposits are molybdenum, tantalum and platinum.

The following is claimed:

l. The method of enhancing photoconductivity in crystalline materialwhich comprises vaporizing the material under vacuumand depositing thevaporized material on a metal: plate carrying oxygen as an activator,said material being selected from the group consisting of suldes,selenides and tellurides of cadmium, indium, thallium, tin, silver,lead, antimony and bismuth and combinations thereof. v

2'. The method `according to claim 1 wherein the vaporized material isdeposited on a metal plate carrying oxygen in the form of an oxide.

3. The method according to claim l wherein the plate is of a metal ofthe group consisting of molybdenum, tantalum and platinum.

4. The method according to claim 1 wherein the activator is oxygenoccluded in the metal plate.

5. The further step in the process of claim 1 which comprisesmechanically removing from vthe plate the photoconductive crystalsdeposited'thereon.

6. The method according to claim 2 wherein the oxide is an oxide of themetal comprising the plate.

7. The method according to claim 2 wherein the oxide is barium peroxideand the plate is of a metal of the group consisting ofmolybednum,tantalum and platinum.

8. The method of enhancing the photoconductivity of cadmium selenidewhich comprises vaporizing crystals of cadmium selenide under vacuum anddepositing the vaporized cadmium selenide on an oxide coated plate ofmolybdenum.

9. The method of enhancing the photoconductivity of cadmium selenidewhich comprises vaporizing crystals of cadmium selenide under vacuum anddepositing the vaporized cadmium selenide on an oxide coated plate oftantalum.

10. The method of enhancing photoconductivity of cadmium selenide whichcomprises vaporizing crystals of cadmium selenide under vacuum anddepositing the vaporized cadmium selenide on a plate of molybdenumcarrying occluded oxygen.

References Cited in the le of this patent UNTTED STATES PATENTS2,448,517 Cashman Sept. 7, 1948 2,600,579 Ruedy June 17, 1952 2,659,678Cusano Nov. 17, 1953 2,732,313 Cusano Jan. 24, 1956 2,810,087 ForgueOct. 15, 1957

10. THE METHOD OF ENHANCING PHOTOCONDUCTIVITY OF CADMIUM SELENIDE WHICH COMPRISES VAPORIZING CRYSTALS OF CADMIUM SELENIDE UNDER VACUUM AND DEPOSITING THE VAPORIZED CADMIUM SELENIDE ON A PLATE OF MOLYBDENUM CARRYING OCCLUDED OXYGEN. 